Only build ipsec modules if the kernel includes IPSEC_SUPPORT.

Honoring the kernel-supplied opt_ipsec.h in r361632 causes builds of
ipsec modules to fail if the kernel doesn't include IPSEC_SUPPORT.
However, the module can never be loaded into such a kernel, so only
build the modules if the kernel includes IPSEC_SUPPORT.

Reviewed by: imp
Differential Revision: https://reviews.freebsd.org/D25059